This invention relates to high intensity arc discharge lamps and more particularly to high intensity arc discharge metal halide lamps having improved color rendition while retaining high efficacy.
Due to the ever-increasing need for energy conserving lighting systems that are used for interior and exterior lighting, lamps with increasing lamp efficacy are being developed for general lighting applications. Thus, for instance, arc discharge metal halide lamps are being more and more widely used for interior and exterior lighting because of their high efficacy, generally good color rendering and high luminosity. Such lamps are well known in being offered commercially in a wide range of lumen output and color temperature, and include therein a light-transmissive arc discharge chamber sealed about an enclosed a pair of spaced apart electrodes which typically further contains suitable active materials such as an inert starting gas and one or more ionizable metals or metal halides in specified molar ratios, or both. They can be relatively low power lamps operated in standard alternating current light sockets at the usual 120 Volts rms potential with a ballast circuit, either magnetic or electronic, to provide a starting voltage and current limiting during subsequent operation.
These lamps typically have a ceramic material arc discharge chamber that typically contains, as active materials, quantities of sodium iodide, thallium iodide and one or more rare earth halides such as dysprosium iodide, holmium iodide and thulium iodide, and also contains mercury to provide an adequate voltage drop, or loading, between the electrodes plus an inert starting gas. Commercially available lamps containing these materials have good performance with respect to Correlated Color Temperature (CCT) and much of the Color Rendering Index (CRI), and have a relatively high efficacy that can be up to 95 lumens-per-watt (LPW). However, such lamps usually do not emit light in the red visible light wavelength range sufficiently to satisfactorily render light for use in illuminating many kinds of scenes as compared to the light provided by incandescent sources for this purpose which closely resembles blackbody radiation at equivalent correlated color temperatures.
The color rendering properties of lamps are indicated by a general index termed Ra which can have associated therewith fourteen further indices representing the color rendering properties of selected test colors provided in CIE Publication 13.2 (1974). One such index designated R9 is the red color rendering index for strong red with Munsell notation 4.5 R 4/13, and a blackbody source has a value of 100 on this R9 index. Even though typical commercially available lamps have a good general color index Ra value, the R9 index value for those lamps will be far less than the 100 value of a blackbody source.
Provision of an adequate red component in the illumination used for lighting indoor scenes is very desirable for use in retail outlets to allow them to properly show their wares and the results of their services not least, in some of the latter instances, because human skin has a better appearance under such illumination. Thus, such outlets that can benefit from such an improved source emission spectrum include restaurants, cosmetic shops, jewelry shops and where fresh food products are offered such as meats, fish, fruits and vegetables. Thus, there is a desire for arc discharge metal halide lamps having better color performance while retaining the hue of the light and the high efficacies typically provided by commercially available arc discharge lamps.